He knew his prince wanted the dagger and if he intends to keep it as his treasure and if it's his will, he will defend it. But if any demons survives, they will follow them to the train, putting everyone on board at risk. So, he'll make it his goal to kill every demon in this world until the mission is over so his prince can keep it. It's not possible. It just takes time.

He carries him to the camp and lay him inside their tent where he could rest properly on the makeshift bedding. "Dianxia, do you want to eat anything? I'll get you something to eat and drink."